1. Field of the Invention
The present invention relates to a control apparatus.
More particularly, the present invention relates to a control apparatus for controlling nip pressure in a nip of a metering size press.
2. Background Information
Conventional metering size press loading systems use a pivoting roll to load the rolls together. With this style design, there are several factors effecting nip loading. These factors include total roll weight, roll to roll weight variations, roll weight variation front to back due to design differences, the weight of the water inside the roll to provide cooling and heating.
In the past, the film coater roll cover was much harder compared with covers used today. Covers used to have a hardness of 10 P&J compared with 40 P&J today. Also the average operating nip level was considerably higher years ago. For example 250 pli compared with 100 pli today. A harder cover operating at higher nip pressures minimizes the influence of the aforementioned factors. A softer cover operating at lower nip pressures is much more sensitive to these factors.
The control system must very accurately measure and control the nip loading. In actual operation this is very difficult to do. The result is inaccurate nip loading, biased nip loading front to back and non-uniform nip pressures when the nip is closed.
The present invention provides a means for controlling the nip pressure without any effect from the aforementioned factors. Therefore, the control of the present invention makes such control much more accurate and provides a more simple method of controlling nip pressure.
The roll that is moved to nip against the fixed roll is mounted on linear bearings. The linear bearings provide a means of aligning the moving roll. Hydraulic cylinders control the nip loading. The following advantages are realized:                The linear bearings have a relatively constant friction value when they are moved. This provides more consistent, accurate nip loading control.        The hydraulic cylinder is smaller which results in lower flow requirements and faster response.        The nip loading is purely a function of the roll geometry (not weight).        Zero pli nip loading=zero psi cylinder pressure. With prior art design, zero pli nip loading was governed by the roll weight+pivot arm weight+friction+other factors. This resulted in the hydraulic cylinder pressure transmitter working over a very small range e.g., a 10% change in nip loading=a 1% change in the control values.        
The front and back posts for the moving roll are mounted on linear bearings. The linear bearings provide an accurate means of positioning the moving roll in the machine and height directions. Hydraulic cylinders, located on the front and back posts, are used to move the posts. The hydraulic cylinders are sized to provide the required nip loading. As such, they can be considerably smaller than existing designs. The design may, or may not, have a cross shaft connecting the front and back posts. The preferred method would not have a cross shaft. The synchronization of the front and back posts can be accomplished with either position feedback devices or a cross shaft. The preferred method is to have three positions for the moving roll as follows:                1/ fully retracted,        2/ ready to load position, and        3/ loaded.        
The ready to load position is with the rolls having a 0.5″ gap.
Load cell feedback of the nip loading can be incorporated into the design. This can be done in many different ways. The two preferred methods are to incorporate a pin type load cell into the hydraulic cylinder or to use a compression type load cell in the front and back framework. Roll weight, external loading, or other factors do not affect the pin load cell design. It is the simplest and cleanest means of providing load cell feedback to the control system. The compression type load cell design will be affected by the roll weight. As such, the signal-to-noise ratio will not be as good as with the pin type design.
Therefore, a primary feature of the present invention is the provision of a control apparatus for controlling nip pressure in a nip of a metering size press that overcomes the problems associated with the prior art arrangements.
Another feature of the present invention is the provision of a control apparatus for controlling nip pressure in a nip of a metering size press that provides more consistent, accurate loading control.
A further feature of the present invention is the provision of a control apparatus for controlling nip pressure in a nip of a metering size press that provides faster response.
Other features and advantages of the present invention will be readily apparent to those skilled in the art by a consideration of the detailed description of a preferred embodiment of the present invention contained herein.